The selection of motor oil for classic Minis The Basics of Lubrication Hydrodynamic Boundary Mixed History

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1 The selection of motor oil for classic Minis By Richard Widman This is the preliminary draft of this paper to help you with the determination of the right oil for your Classic Mini. I m sure there will be additional questions that will arise or points that I have not explained to the satisfaction of some readers. Please send your questions or suggestions to me at oil@asboman.com First edition: May 6, 2018 The Selection of motor oil for our Minis follows closely on one of the principals of Parkinson s Law, which goes something like this: The time spent on any subject is inversely proportionate to its cost, since very few people have expertise in things like designing a space rocket, but everyone has an opinion on motor oil. (He actually takes the discussion from a budget for space travel development down to the level of deciding what brand of coffee to use in the break room of congress). There are at least a thousand oils to choose from, and each of us has a personal preference as to brand loyalty, color of bottle, or viscosity. To be clear, you can put just about any oil in your Mini and not have problems, depending on what you consider a problem. I have people who tell me they use xyz oil and they don t have problems, that with that oil they sometimes get as many as 100,000 km between overhauls. To me, that is a problem. I expect 500,000 km as a minimum. Another concern with the selection of oil is that people often refer to oils by a partial name, or only one of the characteristics. Two people might comment differently on any given viscosity, say, 10W-40, one saying it is too thin, while the other says no. But when you get down to the specifics, once pressed into action in the bearings, or between the gears, one retains 15% more of its body to lubricate with. Some oils shear down much more than others. In many cases a 20W-50 is thinner in the bearings than a 5W-40 or 10W-40. You get what you pay for. There are also people who swear by a specific brand, when that brand has 5 or more offerings of that viscosity, all of different quality and standards. The Basics of Lubrication There are 3 main concepts of lubrication: History Hydrodynamic: This is what the viscosity does, as it creates a cushion of oil that keeps the moving parts separated. Boundary: This is where the additives take action, either through polar or chemical reaction on the metal surfaces when there is no cushion of oil. This is normal in flat tappet valve trains, transmissions, and piston rings, among other places. This is also what lubricates during start-up of the engine until the oil reaches the parts. Mixed: This is the combination of these, when the cushion becomes too thin, which would be normal in many parts of the engine and transmission. When the Mini first came out, the best oils on the market were what we call API SB today. Many brands recommended API MS or DG, which was the most advanced on the market, later to be called API SC (1964). Most people who knew anything about flat tappet valve trains began

2 recommending the addition of ZDDP plus or similar additives, as the SB oils had very few additives, somewhere in the neighborhood of 200 ppm of phosphorous from the ZDDP. There were still problems, and in 1968 new oil formulations were launched with a category we call API SE. I have a Mini service manual that recommends SE oils, ranging from 5W-20 to 20W-50 in viscosity. Thanks to a reader of my previous paper on flat tappet protection, I was able to analyze a can of MS-DG oil from the late 60 s. That analysis showed 517 ppm of zinc and 482 ppm of phosphorous, slightly higher than half the level of today s SN oils, and 30% of today s diesel formula oils. It only had 807 ppm of detergent. So, it is true that just about any oil on today s market is better than what was available when many Minis were produced. And we need to remember that oils were improved to give more protection to the parts, longer change intervals, etc. It is also true that today we expect more of the cars, and many of us drive more sportingly than when they were cheap commuting cars. If you are not interested in the maximum performance and longevity, you can skip to the summary. First things first The first thing we should consider is the Owner s Manual that came with the cars when they were new. And just in case there are different recommendations in the service manual, we should check them as well. Finally, we should consider developments in oils over the last 60 years, as most of today s oils do not shear like older oils, whether mineral or synthetics. To begin with, we should consider that the design of the engine, with its transmission, whether manual or automatic, is attached and using the same oil. This will affect the oils we choose. And keep us from using the oils we use in other cars. Design The Mini engines, as all engines, have a bypass valve that diverts excess oil pressure and flow back to the oil pan when it reaches more than the pressure it needs, preventing rupture of filters or O-rings, and maintaining enough oil in the pan instead of having excess waiting in the valve cover to return by gravity. According to the various manuals I have, The Mini started out with this at 60 psi, later using 70 psi, then 50 psi with the Cooper, and finally, with the SPI and later, 30 psi. This bypass valve should not be confused with the one in the oil filter or it s housing, as that one is there to guarantee the flow of oil to the bearings, when the oil is too thick to push through the filter paper or the filter is clogged from soot or particles. The most important factor in lubrication of an engine is the viscosity. It needs to be strong enough to hold its cushion in the bearings and keep the crankshaft from the rod bearings. But it also has to be thin enough that it can get there quickly. The thicker the oil, the more wear you have every time you start the engine. There are three basic areas that we need to examine in order to judge the right viscosity, considering that we may be starting the engine at freezing temperatures and driving them at engine temperatures of 90ºC or more. The selection of motor oil for classic Minis. 2

3 To understand the following dimensions, I will compare them to something we can relate to: A human hair, which has a nominal diameter of about 80 microns (µm). it is also worth noting that the human eye can only see things that are larger than 40 µm. Looking at the following charts from the manuals, we see that our clearance in the rod bearing varies from 12 µm to 69 µm on the 85H engines, and 25 µm to 51 µm on all the rest. So, we need oil that will flow into spaces one-sixth to one-third of the width of a human hair when the engine is properly rebuilt, increasing with wear to two-thirds (or so) of that width on a worn engine. We should also note that the main bearing clearances basically fall within the same ranges, so no adjustments need to be made. Looking at the piston skirt-to-cylinder clearance, we see that the 85H, 99H, and 10H engines have less clearance than the bearings. This means that if the oil is too thick, it needs to get pushed down by the piston skirt instead of lubricating the cylinder walls and flowing out the oil control rings. We should also note that these clearance specs are very similar to the BMW Minis, where 0W- 30 and 0W-40 oils are recommended. Obviously, those do not have to concern themselves with the transmission parts and forces. The selection of motor oil for classic Minis. 3

4 Note that the control rings have very small openings, as do the pistons. In addition to these small groves there is a 38 to 89 µm gap between them and the piston groove. Oil that is too thick, and does not flow well, does not cool the pistons, and often burns in these grooves, building carbon and sanding the cylinders. In the following pictures we can see the way this carbon builds up, and the small passageway that it has to flow through to take the engine heat back to the sump. Carbon filled groves in SPI piston Actual drain holes behind oil control ring Valve train We should also consider how long it takes for the oil to get from the pump in the gear box to the valve train on top of the engine. The noises caused by this delay can be very noticeable for a few seconds of running dry on engine starts. Noise equals damage. If you ever get a chance, watch how a 20W-50 comes up to the valve train and just flops over, not spraying around all the parts like it should. Then put a 10W-40 in and watch how it sprays everywhere. Manual Transmission We also need to consider that this oil must lubricate and protect the transmission. In the case of the manual transmission, the Mini uses needle bearings and roller bearings. The best lubricant for needle bearings is relatively thin, so that it can flow into those small areas between the needles. The best for roller bearings is often thicker, depending on the size and type. The design and size of the synchronizer cones is also important, as thick oil is much The selection of motor oil for classic Minis. 4

5 harder to displace than thin oil. Synchronizer design and needle bearings are why most cars since about 2005 have recommended 75W-80 and 75W-85 oils, or ATF in their manual transmissions, while most cars produced from the mid 60 s until then recommended 75W-90 oils. We will use 75W-90 oils for our viscosity comparisons, below. Automatic Transmission The AP automatic transmission used in the Mini is very sensitive to viscosity since the grooves on the discs are very shallow. The oil must displace quickly. And It should have additives to make the discs grab each other instead of slipping or chattering. If today s gasoline oils, meeting API SN or the latest ACEA specs are used in this transmission, the discs will slip and wear out rapidly, just like slipping the clutch of a manual transmission car. While diesel formulations do not have those slippery friction modifiers, and are therefore better, the Motorcycle MA2 oils have a special friction modifier that make them grab between 35% and 100% better than diesel formulations (depending on the brands). MA2 oils will give noticeably better shifting and less particles in the oil and filter when you change it. Additives and formulations The oil we put in the engines of our Minis must fight off byproducts of combustion, protect sliding flat tappet valve trains, protect needle and roller bearings in the transmissions, and provide a certain level of EP protection for the gears, while allowing synchronization in the transmission. Obviously, we will never consider putting a standard transmission oil in our Minis, since their formulation with sulfur/phosphorous EP additives would cause serious damage to the engine parts and form acids instead of neutralizing them. But should we put current friction modified API SN gasoline engine oils in there? The answer is no, for two basic reasons. First, just like any flat tappet engine, with the sliding action of our valve trains, we are better protected by ZDDP than the modern friction modifiers designed for roller bearing rockers and various other things. With valve spring pressure originally 70 lbs. and then 94 lbs. (not as high as some engines, but higher than many of that time.), we need that ZDDP. And second, because they could be too slippery for the synchronizers. The manual transmission Minis are best served by diesel formulation oils since they have higher levels of ZDDP. The ZDDP does a better job of protecting the valve train, and most of them will provide between 45 and 50 lbs. of protection in the Timken tests, covering the GL-4 manual The selection of motor oil for classic Minis. 5

6 transmission requirements. We can also use some special classic car oil formulations, although they are not necessary. For those that are concerned with this recommendation for diesel formulations, I recommend you check out this paper on the subject where the testing is explained, rather than repeat most of those 30 pages here. Note that ZDDP is a great anti-wear additive, composed of zinc and phosphorous. But too much will cause galling to the cam and other steel parts under pressure. SAE studies show that 1800 parts per million (ppm) is the maximum limit most steel will withstand without damage ppm has been shown to cause serious damage. This means that it is very dangerous to put additional additives in any oil, as you would need to know the formulation of that specific oil and the additive we are considering, then calculate the amount to use. Diesel formulations generally contain between 1200 to 1400 ppm of zinc and phosphorous from ZDDP, quite sufficient for the protection we need. They have more detergent than we require, but that does not hurt anything. I have seen classic oils offered that range in phosphorous from 1200 to The latter of those would cause damage to the cam and possibly gear teeth. Don t be fooled by the mention of ZDDP in the oil. One Classic Car Oil says It is formulated with ZDDP (zinc/phosphorus). On their web site and spec sheet. All motor oils (except SA) are formulated with some amount of ZDDP. Viscosity Viscosity is the resistance of a fluid to flow. The higher the resistance, the higher the viscosity number. The higher the viscosity, the more heat is generated by its movement. Viscosity is a term that is not well understood. Oil is a non-newtonian fluid. That means its thickening or thinning is not directly related to the temperature. Many people feel the viscosity of the oil in their fingers and assume it is like that in the engine. It thins down severely at engine operating temperatures, and thickens at colder temperatures. Secondly, there are four viscosity scales. Two are automotive scales determined by the Society of Automotive Engineers (SAE). One of these is for engine oils, while the other is for gear oils. In general, we can think of a SAE 50 engine oil being the same viscosity of a SAE 90 gear oil. In the following graph, we can see how an assortment of oils work at different temperatures. This first graph starts with 0ºC (32ºF) and shows the relationship of various transmission and engine oils as they heat up in use. Note that at 0ºC, a 20W-50 is almost three times thicker than 10W-40, being very difficult to pump and lubricate in both the engine and the transmission. It shows that if anyone thinks they need a SAE 50 for high temperatures, they would be much better off with a 5W-50. Note that we are not talking about Density. If we were to add a line for the Density, it would be a straight, sloping line, and each oil would have its own line, depending on the density of the base oil and additives. Note that the two gear oils are only shown for relative viscosity. They should never go in a Mini. The selection of motor oil for classic Minis. 6

7 The selection of motor oil for classic Minis. 7

8 In this second graph, we see how these same oils flow at higher temperatures, as they begin to heat up and operate in the engine and transmission. The selection of motor oil for classic Minis. 8

9 When we look loser at the viscosity in the hotter parts of the engine, eliminating the gear oils from our graph, we can see how the three examples of xw-50 oils converge to the same viscosity, albeit much thicker than the 10W-40 in our examples. The reason to graph these oils is to see the effect on the temperature on the oil. If we think, for example, that we should have about 18 cst at 90ºC, why would we use oil that only thins down to 24 cst at that temperature? Or, if we think we should have an oil that is 24 cst at 90ºC, why would we use a 20W-50 that is 2900 cst (120 times thicker ) at freezing temperatures and 170 cst (7 times thicker ) at 40ºC? The selection of motor oil for classic Minis. 9

10 Recommendations in manuals It is interesting to note the variations in owners and service manuals. Here is what is published by Austin/Morris/Rover. In this first table, we see recommendations (ignoring the column for really cold climates) of 10W-30, 10W-40, 10W-50, and 20W-50. It should be noted that oils of 50 years ago had serious problems with viscosity shear and high concentrations of unsaturated molecules that collapse under pressure, leading many manufacturers to recommend thicker oils, especially when they would be in contact with gears, which tend to mill, or grind, the polymers used to form multigrade oils. Today s group II base oils need less polymers to maintain their viscosity and body, and today s polymers are generally much more resistant to shear, although they vary in quality with the price paid. The better synthetic oils do not use polymers to be multigrade. Cheaper synthetics will contain polymers, although they should be the best available. The selection of motor oil for classic Minis. 10

11 In another publication they show the viscosities the way many brands used to do (and still do for the third world). While there is some logic to it, we should consider what can be used, and what is best to use. According to this chart, you could use any oil that is 10W-30 up to 10W-50 if your lowest startup temperature is -20ºC. It also then suggests that 15W-40 up to 20W-50 are suitable for startup temperatures of 0ºC. The SAE J300 standards have changed a bit over the years, but, as we see in the next chart, the oils are not tested at the same temperatures, as they are not expected to behave the same way. Oils that claim 15W, cannot thicken to more than 7,000 cp at -20ºC, and oils that are 20W must not thicken to more than 9,500 cp at -15ºC. So, these two oils have very different cold flow properties and should not be considered similar. Note that we are using centipoise (cp) on this test instead of centistokes (cst). The conversion depends on the density of the oil. Viscosity in centistokes will be 10% to 15% higher. Centipoise is also used to measure the actual film strength in the bearings, at 150ºC, under pressure, where a xw-30 must maintain 2.9 cp, an xw-40 must maintain 3.5 cp), and an xw-50 must maintain 3.7 cp in the ASTM High-Temp/High-Shear test. I have a 5W-40 synthetic that maintains 4.0 cp in this HTHS test (and a group II SAE 15W-40 with 4.2 cp). So, with the right 10W-40, you can have a physically thicker cushion in the bearings than with many 20W-50 oils. The HTHS test is the most critical for bearings and for our transmissions, as it shows how thick the actual fluid barrier is in those places under pressure. Here are a couple more pieces of manuals, this time from The owners manual recommends 10W-30, although notes that this is up to 30ºC. The service manual says 10W-40. The selection of motor oil for classic Minis. 11

12 The 1992 service manual The 1992 Owner s manual (note that it says up to 30ºC) Summary The decision of the right oil takes you to a diesel formulation (API CI-4 or later) or a specialty oil if you want to take care of your valve train and gears. You do not want a common gasoline formula oil, such as SN. If you have the automatic transmission, you need a JASO MA2 oil. For viscosity, take a good look at the graphs and take a look at how fast those curves take you down into the viscosity range where it can circulate and protect. For most recently rebuilt engines, the manuals and graphs show that a 10W-40 would lubricate faster and better, but I would look for one with a high HTHS so that you have the best of both worlds: circulation and shear strength. But if you feel you need something thicker, I recommend a 5W-50 instead of 20W-50, as it will flow much better on startup and have a much stronger film thickness (HTHS) in the bearings. I have had people say as long as it has the API certification it is fine. While that is a basic truth, in our Minis, shear strength is very important in the gears. So, where an xw-40 must not shear down below 3.5 cp, which is sufficient for the engine, why not the better ones with 4.0 or more? These are made from group II base oils or better. There are also some oils that meet the standards when new, but shear rapidly in use. I have seen several that do not last more than 2000 km. It should be noted that since almost no car manufacturer (I know of only one, in India) recommends 20W-50, it is an oil normally sold on price, not quality. You may be getting something that will shear down to below a good 10W-40. There is a highly advertised specialty The selection of motor oil for classic Minis. 12

13 20W-50 oil for classic cars that is made from a group I base oil. That is the bottom of the pile in base oils. On viscosity, remember there are two important points: How well it flows on startup and until it reaches operating temperature. The physical film thickness when it is hot and compressed in the bearings (HTHS value). At one time, the API was looking to require the HTHS value on the labels. This idea was killed with the excuse of being too complicated for the consumer. For European oils, the ACEA standards do not identify anything over 3.5 cp on the HTHS, but to have that final high shear value of 3.5 or greater you need a C3 or C4 ACEA rating. While I, like all car manufacturers, recommend never using an aftermarket additive in the oil, there is, in my opinion, one exception to this rule. I have fixed the compression and leaking of many engines by driving about 2000 km with an Ester based additive that dissolves carbon and sludge, while softening and swelling dried seals. While I have my own brand of this, I will not mention it by name, as this is not an advertisement. The most recognized and available product on the market is AutoRx, sold over the internet. At this point, most Minis on the road have been repaired to some extent. There is no perfect oil for all Mini s in their current condition, so here are some things to consider (covered in detail above): Never use an API SN rated oil. If you have the AP automatic, your best performance and least wear will normally be with a 10W-40 JASO MA2 motorcycle oil. If you have a manual transmission, find an oil with at between 1200 and 1600 ppm of phosphorous. This will either be a diesel rated oil or a specialty oil. Never put additives in the oil you buy. Very few people can calculate and improve the formula. For those that want more detail on why, check this link on the development of the standards. If you have low oil pressure (measured between the pump and bearings), you should use a thicker oil until you replace the bearings or correct the pump. If you have low compression, you may want to increase the oil viscosity slightly to compensate until you rebuild, although you should consider that this will cause higher valve train and bearing wear until that time. If you have worn main shafts in your transmission, you should use a thicker oil to compensate for that wear. If you have poor synchronizer action, you should consider reducing the viscosity (unless you are using a SN formulation that should be changed out first). If you decide to stay with the faster flowing 10W-40, you can certainly use a 5W-40 synthetic. It will probably a higher HTHS viscosity in the bearings and gears than a mineral 10W-40. It is also easier to find in a diesel formulation. If you decide you need an xw-50, seriously consider a synthetic, preferably 5W-50 or 10W-50. You will have much better startup flow and protection. Remember that the 50 in that spec is the flow resistance. The HTHS value is much more important. The selection of motor oil for classic Minis. 13

14 But whenever any engine is rebuilt, no matter the car, you should go back to factory spec viscosity. In the case of the Mini, not rebuilding the transmission at the same time might affect that procedure. I would like to conclude this with a list of recommended oils, but that is not possible, since formulations vary from country to country, even within the same brand and name. I know of excellent, group II base stock oils produced in some countries by certain brands, while they use cheap group I base oils in another country, yet put the same product name on the oil. What I can recommend, is that after reviewing these factors, when you look at the shelves full of oil, or the internet, look first at the all the oils of viscosity you have chosen that are CI-4, JASO MA2, Classic, or whatever you determined, and if possible check their HTHS and base oil. The last thing to look at is the brand name. I will add that, personally, I use a Semi-Synthetic JASO MA2, 10W-40 in my SPI. I rebuilt it to standard specs about 1000 km ago (as of the initial writing of this paper). And one final recommendation: When someone asks what oil to use, remember the points in this summary, and avoid the pitfalls I mentioned at the beginning. I hope this is helpful. If you have questions or comments, feel free to write to me at oil@asboman.com The selection of motor oil for classic Minis. 14